Furnace brazing generally refers to processes for joining together metal parts which have a melting point appreciably above the filler joining metal. The metal parts are prepositioned onto an assembly which is passed through the brazing furnace wherein preformed brazing material is melted and drawn into a close fitting joint by capillary action to make a strong brazed joint. Copper is the most frequently used filler metal for furnace brazing. More recently, brazing pastes have been developed for brazing metal parts together. Brazing pastes are metal powders most often copper and copper oxide dispersed in a vehicle to provide a paste consistency which can be easily applied to any shaped metallic joint which eliminates the need for a variety of preformed brazing parts. Examples of typical brazing pastes may be found in a series of Klinker patents such as, for example, U.S. Pat. No. 2,566,339; U.S. Pat. No. 2,594,313; and U.S. Pat. No. 2,606,132; and the disclosures of said patents are incorporated herein by reference.
The formation of a properly brazed joint is dependent on a number of factors (AWS Brazing Manual) central to which is the melting temperature of the brazing alloy. This temperature for copper is about 2050.degree. F., while certain alloys of copper are lower. Lower brazing temperatures are desirable since these provide advantages in part fixturing, economy in furnace brazing, and improved life of the furnace and furnace accessories. While the melting temperature of a copper-manganese alloy makes it a candidate for reduced temperature brazing, rapid oxidation of the manganese under normal brazing conditions has prevented its commercial use.
It has now been found that a copper alloy brazing paste composition can be produced from a copper-manganese alloy containing a minor amount of silicon up to about 3% by weight. The inclusion of silicon prevents oxidation of the copper alloys since the silicon appears to migrate to the surface of the alloy particles during water atomization process for producing such particles. The silicon is preferentially oxidized with respect to either the copper or manganese matrix metal whereby an oxide film is orientated toward the particle surface. The oxide film can be effectively and easily removed during brazing as distinguished from copper oxides or manganese oxides distributed throughout the particles. These and other advantages of this invention will become more apparent by referring to the Detailed Description of the Invention.